Vibration isolation systems are known, and are required in many technological sectors, for example in the semiconductor industry. DE 69817750 T2, for example, discloses a vibration isolation system which is provided in particular for vibration-isolated support of a lithographic device. A load to be supported, which typically includes a platform and components supported thereon, for example production equipment, is supported on air bearings.
In addition to a support having the lowest possible mechanical rigidity, active vibration isolation systems have sensors and actuators which in a targeted manner counteract external vibrations which penetrate the system. The sensors detect motions of the load being supported. By means of a control device, compensation signals are generated which control actuators and thus generate compensating motions. In this manner it is possible to use digital or analog control loops, or to use both together, referred to as hybrid control loops.
The demands on vibration isolation systems are becoming ever more stringent with increasing miniaturization in the semiconductor industry. In the prior art, so-called geophones are typically used as sensors to record the mass movements.
A geophone is an electromechanical transducer which converts the detected vibrations into analog and/or digital signals. In general, a geophone essentially comprises a coil and a permanent magnet, the coil being connected to the permanent magnet via a spring. When a relative motion caused by vibration occurs between the coil and magnet, a voltage is induced in the coil which is proportional to the velocity of the motion. Geophones are fairly inexpensive, and by use of suitable measures have a frequency response characteristic which extends to very low frequencies, in particular as low as 0.1 Hz or even lower. In contrast, at high frequencies the bandwidth of the frequency response characteristic for a geophone generally extends only to approximately 300 Hz, which in most cases is adequate for vibration isolation. However, detection of higher frequencies using the geophone itself is not possible.